Several new features are added for the long term High Speed Packet Access (HSPA) evolution in order to meet the requirements set by the International Mobile Telecommunications Advanced (IMT-A). The main objective of these new features is to increase the average spectral efficiency. One possible technique for improving downlink spectral efficiency would be to introduce support for four branch multiple-input multiple-output (MIMO), i.e. utilize up to four transmit and receive antennas to enhance the spatial multiplexing gains and to offer improved beamforming capabilities. Using four branch MIMO currently provides up to 84 Mbps per 5 MHz carrier for high signal-to-noise ratio (SNR) users and improves the coverage for low SNR users.
The current High Speed Downlink Packet Access (HSDPA) system (Release 7-10) supports up to two (2) branch MIMO, i.e. supports up to two (2) transmit antennas at the network node. For these HSDPA systems, from channel sounding, a user equipment (UE) measures the channel and reports the channel state information (CSI) in one sub-frame. Typically, this report comprises a channel quality indicator (CQI) which explicitly indicates a Rank Indicator (RI) and a Precoding Control Indicator (PCI). The UE sends this report periodically for every subframe, i.e. for every Time Transmission Interval (TTI). The reporting here uses the MIMO codebook for 1-2 transmit antennas according to Release 7 MIMO. Once the network node receives this report, the network node grants the modulation and coding, the number of codes, the rank and the precoding channel indicator to each specific UE based on the scheduler metric.
However, with the introduction of four (4) branch MIMO in a HSPDA system, i.e. up to four (4) transmit antennas at the network node, a new feedback channel structure to send the CQI/PCI information to the network node from the UE is required. This is because in a four (4) branch MIMO system using two codewords, there is up to 4 transport blocks of information that may be processed simultaneously. This also means that there will be up to 4 ACK/NACK in the HARQ signalling, since each transport block is decoded and CRC (Cyclic Redundancy Check) check is applied at the network node.
Since this information is to be represented in the first slot in a sub-frame of the uplink signalling channel, i.e. the High Speed Dedicated Physical Control CHannel (HS-DPCCH), a MIMO codebook for the four (4) branch MIMO is needed to represent the ACK/NACK information in the HARQ signalling.
Reference [1] discusses the possibility to introduce larger transport block sizes so that there would be two transport blocks within a new larger transport block, and use the Rel-7 HARQ-ACK codebook.
Reference [2] also briefly mentions the possibility to re-use the Rel-7 HARQ-ACK codebook for 2 codeword, 2 HARQ processes in the HARQ design for a four branch MIMO system; but without mentioning how this should be done.
There is thus a general need for an efficient way of encoding Hybrid Automatic Repeat Request, HARQ, Acknowledgements, ACK/NACK, in a multi-antenna wireless communications system. This will for example be useful in MIMO systems and similar communications systems.